Measurements of light atmospheric hydrocarbons over the Atlantic in regions of low biological activity

Abstract

More than 200 in situ measurements of several selected nonmethane hydrocarbons (NMHCs) were made in the remote marine atmosphere over the Atlantic between 40øS and 50øN. Ethane, the by far longest lived of the NMHCs, showed southern hemispheric mixing ratios around 280 ppt, comparable in magnitude to most other previous measurements. The mixing ratios of ethene, propene, propane, and i-and n-butane in the southern hemisphere were in the range of 10-30 ppt. For i-pentane and n-pentane the atmospheric mixing ratios in the southern hemisphere were below the detection limit of 15-20 ppt. These values are rather low compared to other published measurements in the marine atmosphere. This is due to the remoteness of the measuring locations and the very low biological activity in the surrounding ocean areas. For all alkanes the latitudinal profiles exhibit a considerable decrease from north to south. The relative hydrocarbon patterns show that the alkanes in the northern hemisphere are primarily due to long-range transport from continental or coastal areas. In general the transport times exceed several days. Consequently, the observed northern hemispheric ethene and propene mixing ratios must be, with few exceptions, primarily the result of oceanic emissions. The average difference of these compounds by a factor of 2 between the southern and the northern hemisphere can be explained by change of the phytoplankton concentration in ocean water. There is little or no indication for the existence of significant diurnal cycles for these two alkenes.

The distribution of light nonmethane hydrocarbons over the mid-Atlantic: Results of the Polarstern cruise ANT VII/1

“…On the other hand, for March, they report ethane mixing ratios up to 2.5 ppb in the Northern and less than 200 ppt in the Southern Hemispheres. This agrees with our Atlantic data for March/April 1987 (Rudolph and Johnen, 1990) and September/October 1988 (Table I). Another data set showing the seasonal variation of ethane in the Southern Hemisphere is published by Rudolph et al (1989).…”

“…Results of recent measurement series (cf. Rudolph and Johnen, 1990, and references therein) confirm that many light NMHC have substantial oceanic sources. Measurements in the surface water of the ocean show that light NMHC are supersaturated relative to the atmosphere (e.g.…”

“…Comparison of the long-lived species like ethane and acetylene with a previous measurement series at a different season (cf. Rudolph and Johnen, 1990), gives evidence for seasonal variations in both hemispheres at practically all latitudes with minima in the respective late summer and maxima in late winter.…”

“…With increasing southern latitudes ethane showed a quite steady decrease to about 350 ppt at 300 S. There are a number of measurement series of ethane in the marine background atmosphere, from over the Atlantic and over the Pacific (Bonsang and Lambert, 1985;Blake and Rowland, 1986;Ehhalt et al, 1985;Rasmussen and Khalil, 1982;Rudolph and Ehhalt, 1981;Rudolph et al, 1982;Rudolph et al, 1884;Rudolph and Johnen, 1990;Singh and Salas, 1982). However, in order to compare the different latitudinal distributions, we have to consider the existence of substantial seasonal cycles of ethane both in the Northern and in the Southern Hemispheres (Blake and Rowland, 1986;Rudolph et al, 1989).…”

“…While the large-scale distributions of CO and CH 4 are reasonably well known (cf. Schmidt et al, 1982), most of the NMHC show a considerable variability in their tropospheric mixing ratios (Rudolph and Ehhalt, 1981;Rasmussen and Khalil, 1982;Singh and Salas, 1982;Rudolph et al, 1982;Rudolph et aI., 1984;Bonsang and Lambert, 1985;Greenberg and Zimmermann, 1984;Ehhalt et al, 1985;Rudolph, 1988;Rudolph and Johnen, 1990). The mixing ratios of the longer-lived compounds exhibit a number of systematic variations, whereas the distribution of the shorter-lived species show a considerable variability in these measurement series.…”